Optical waveguide devices are indispensable in various high technology industrial applications, and especially in telecommunications. In recent years, these devices, including planar waveguides, and two or three dimensional photonic crystals are being used increasingly in conjunction with conventional optical fibers. In particular, optical waveguide devices based on chiral optical fibers are advantageous and desirable in applications in which conventional optical fibers are also utilized. Such chiral fiber devices are disclosed in the following commonly assigned, issued U.S. Patents, and co-pending commonly assigned Patent application, all of which are hereby incorporated by reference in their entirety: “Chiral Fiber grating” (U.S. Pat. No. 6,839,486), “Chiral In-Fiber Adjustable Polarizer Apparatus and Method” (U.S. Pat. No. 6,721,469), “Chiral Fiber Sensor Apparatus and Method” (U.S. Pat. No. 6,792,169), “Customizable Chirped Chiral Fiber Bragg Grating” (U.S. patent application Ser. No. 10/311,447), “Chiral Broadband Tuning Apparatus and Method” (U.S. Pat. No. 7,009,679), “Customizable Apodized Chiral Fiber Grating Apparatus and Method” (U.S. Pat. No. 6,741,631), “Extended Chiral Defect Structure Apparatus and Method”, (U.S. Pat. No. 7,142,280), and “Long Period Chiral Fiber Grating Apparatus, (U.S. Pat. No. 6,925,230).
However, there are significant challenges in interfacing optical waveguide devices, including chiral optical fiber devices, with conventional low index contrast optical fibers. Typically, at least two major obstacles must be dealt with:    (1) the difference between the diameters of the optical waveguide device and the conventional fiber (especially with respect to the differences in core sizes), and    (2) the difference between the numerical apertures of the optical waveguide device and the conventional fiber. Failure to properly address these obstacles results in increased insertion losses and a decreased coupling coefficient at each interface.
While attempts have been made to address the difficulties of interfacing between different optical fibers, as well as between optical fibers and signal sources, the proposed solutions do not address the challenge of mismatched apertures. As a result, the connected optical fiber becomes undesirably multi-mode. For example, the U.S. Pat. No. 4,877,300 to Newhouse et. al., discloses a tapered connector, for use with optical fibers and light sources connectable to optical fibers, that is purported to be less sensitive to misalignment. However, the approach proposed in the Newhouse patent increases the waveguide diameter of the connector. As a result, the connector becomes multi-mode, and therefore loses the capability of maintaining a single, or a predetermined small number of modes which may be a key requirement in interfacing optical waveguide devices (e.g., planar waveguides, or chiral optical fiber devices) with conventional low-index-contrast optical fibers. Furthermore, the connector disclosed in the Newhouse patent does not provide a solution for the challenge of mismatched apertures of optical waveguide devices and conventional optical fibers.
It would thus be desirable to provide an optical fiber coupler that provides a high coupling coefficient interface between an optical waveguide device having a high numerical aperture and a conventional optical fiber having a low numerical aperture. It would further be desirable to provide an optical fiber coupler having configurable characteristics for interfacing with optical waveguide devices and optical fibers of different sizes and characteristics. It would also be desirable to provide an optical coupler that is capable of substantially maintaining a single mode or a greater predetermined number of modes therein. It would additionally be desirable to provide an optical fiber coupler that can be easily and inexpensively fabricated. It would also be desirable to provide an optical fiber coupler that is capable of being fabricated as part of an optical waveguide device.